Regulation of phytoplankton dynamics by vitamin B 12

Despite the biological necessity of vitamin B 12 (cobalamin), its importance in phytoplankton ecology has been ignored for nearly three decades. Here we report strong and selective responses of phytoplankton communities to varying low levels (5–87 pM) of dissolved B 12 in several coastal embayments. The ecological importance of this vitamin is inferred from observed declines in dissolved B 12 levels as field populations of large (>5 μm) phytoplankton increased. In contrast, biomass of small (<5 μm) phytoplankton varied independently of B 12 concentrations. These observations were corroborated by field‐based nutrient amendment experiments, in which B 12 additions stimulated growth of large phytoplankton taxa 6‐fold over unamended controls. In contrast, small taxa (<5 μm) were largely unaffected. This study provides the first evidence of vitamin B 12 's influence on phytoplankton field population dynamics based on direct chemical measurements of cobalamin, and implicates B 12 as an important organic regulator of photoautotrophic fertility in marine systems.